This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Recent crystallographic characterization suggests unique [4Fe-4S] cluster coordination in the active site of the sulfatereducing enzyme, APS reductase. However, the modest resolution of the structure (2.7 [unreadable]) does not permit detailed analysis of cluster geometry, which includes coordination by two invariant adjacent cysteine residues. XAS will be used to provide detailed local structure information for the unique [4Fe-4S] cluster in APS reductase.